Cellular radio communication systems typically include a number of central communication base sites. Each central communication site has a service area coverage for servicing mobile communication units within the service area. The service areas typically are arranged such that adjacent remote base site service coverage areas overlap in a manner that provides a substantially continuous service region. The substantially continuous service region provides uninterrupted service by handing off mobile communication units from one base site serving a service area to an adjacent base site serving another service area.
Communication between the central communication sites and mobile communication units typically occurs on a pair of frequency channels (i.e., transmit and receive frequencies) assigned according to a cellular communication system channel reuse plan. Upon activation, a mobile communication unit searches a radio frequency spectrum for control signal trasmissions from a local central communication site. The control signal transmissions from the local communication site are found in a radio frequency broadcast control channel (BCCH). The BCCH contains specific informationneeded by the mobile communication unit in order to format and code radio communication system access requests to the local central communication site. The Group Special Mobile (GSM) Pan-European cellular communication system, as specified in GSM recommendations by Motorola available from the European Telecommunication Standards Institute (ETSI) and incorporated herein by reference, is an example of a system using such a format including the BCCH.
On the BCCH many control tranmissions are transmitted including timing information, a local central communication site ID for the central communication site which is transmitting the information, format information for specifying the format of transmit access requests, and may further include in some environments information identifying frequencies on which to transmit such access requests. In addition, the formatting information may further identify the communication system as a time division multiple access (TDMA) system and may identify a time slot in which to transmit an access request.
The mobile communication unit upon detecting and decoding information received on a BCCH subsequently transmits an access request to a local central communication site. The local central communication site, upon receiving the access request from the mobile communication unit subsequently responds by transmitting a signal directed to the mobile communication unit which identifies a resource which it can use for communication with the local central communication site.
To limit noise in cellular communication systems due to communication between other mobile communication units in other nearby service areas serviced by other central communication sites as well as increase the capacity of the cellular communication system, reuse of the available, but limited number of communication resources is done within a service region of the cellular communication system. To ensure that the reuse of communication resources does not cause unacceptable noise in the communication channel, central communication sites which are allocated the same communication resources are geographically separated. By having sufficient geographic separation, the noise in the communication channel is limited. However, the geographic separation needs to ensure an adequate signal to noise ratio (negligible noise in the communication channel) limits the capacity of the communication system because not all of the available communication resources may be used in each service area.
To enhance the efficiency of communication resource reuse and to improve capacity of the cellular communication system, service areas of central communication sites can be divided into sectors, wherein each sector a percentage of the available communication resources, (i.e., communication channels). By having the service area divided into sectors, the required geographic separation may be reduced while maintaining an adequate signal to noise ratio. For example, U.S. Pat. No. 4,128,740, assigned to Motorola, Inc. discloses a four cell (service area)--six sector communication resource reuse pattern. As disclosed, each cell is divided into six sectors and each sector contains approximately 1/24th of the available communication resources. For every four cell sites, the communication resource pattern is repeated. This communication resource reuse pattern may be further reduced to a 1 cell site reuse pattern as disclosed in pending U.S. patent application Ser. No. 07/459,624 which was filed Jan. 2, 1990 and also assigned to Motorola, Inc. It will be appreciated by those skilled in the art many other reuse patterns exist for use in cellular communication systems including but not limited to 3, 7, 21, 49, 63, 91 site reuse patterns.
After allocating a communication resource for use by the mobile communication unit, the local central communication site typically allocates a transceiver typically located at the local central communication site to service any subsequent communication with the mobile communication unit on the assigned communication channel resource. The transceiver of the central communication site will subsequently route the communications of the mobile communication unit to a target communication unit. The target communication unit maybe either another mobile communication unit within the same service area, a mobile communication unit in another service area, or a subscriber on the public switched telephone network (PSTN).
High density cellular communication systems with sufficient spectrum can serve a large number of users in an urban environment by using microcells which service a small geographic area. However, in lower user density like rural or suburban areas, higher signal propagation losses and high infrastructure costs severly limit the economic viability of such communication systems. Since providing service coverage to rural, suburban and urban environments alike is crucial in establishing a complete cellular service, the need for a less costly means of deploying a cellular communication system is large.
Another consideration is that due to increasing competition for frequency spectrum, the service coverage areas of central communication sites may have to be reduced to allow for frequency reuse in a relatively close proximity. Reducing the service coverage area of a central communication site, on the other hand, increases the number of central communication sites necessary to cover a given geographic area. Therefore, it is desireasble to minimize the cost of the individual central communication sites equipment so that the overall cost of the cellular communication system can be reduced.